Batteries for use in hybrid and electric vehicles are known from the prior art, said batteries being referred to as traction batteries since they are used to feed electrical drives.
In previous patent applications from the applicant, drive systems which in each case have a battery system with an output voltage that can be set in a stepped fashion and which are used nowadays, for example, in electric and hybrid vehicles or in stationary applications, such as in the case of rotor blade adjustment in wind turbines, are described. An example of such a battery system is illustrated in FIG. 1.
A battery system 100 comprises a battery 101 which is connected to a DC voltage intermediate circuit 102 which comprises a capacitor 103. The DC voltage intermediate circuit 102 is integrated in a pulse-controlled inverter 104 contained in the battery system 100, via which pulse-controlled inverter sinusoidal voltages, which are phase-shifted with respect to one another, for the operation of a three-phase electric motor (electric drive motor) 105 are provided at three outputs in each case by means of two switchable semiconductor valves (not marked) and two diodes (not marked). The capacitance of the capacitor 103 is in this case designed to be sufficiently large in order to stabilize the voltage in the DC voltage intermediate circuit 103 for an interval in which one of the switchable semiconductor valves is turned on.
The battery 101 comprises a battery module string 106 with a plurality of series-connected battery modules, of which only two battery modules 107, 108 are illustrated in the drawing. A charging and disconnecting device 110 is connected between the battery module 107 and a positive pole 109 of the battery module string 106, which positive pole in this case forms a positive battery terminal. Optionally, a disconnecting device 112 can additionally be connected between the battery module 108 and a negative pole 111 of the battery module string 106, which negative pole in this case forms a negative battery terminal. The battery modules 107, 108 can have coupling devices (not illustrated) by means of which they can be selectively separately connected in each case to the battery module string 106 or decoupled from the battery module string 106, that is to say can be bypassed, in order thus to implement a battery system with an output voltage that can be set in a stepped fashion.
Battery management systems are used in traction batteries in order to ensure the safety of such a battery system, to exploit the capacity of the battery cells and to increase the service life of the battery cells. The core function of said battery management systems is a so-called battery state detection, which determines the present state of the battery cells. One of the essential items of information in this case is the internal resistance of the battery cells, which is determined as a function of the state of charge, the temperature of the battery cells and their state of aging. Such a method for characterizing battery cells is described in the initiative of the German Association of the Automobile Industry “Energy Storage System for HEV” and in the draft of ISO 12405.
The basic principle of said method is illustrated in FIG. 2. In this case, FIG. 2 shows exemplary voltage changes of a battery voltage UBatt which is evaluated during a pulsed charging and discharging current IBatt of the battery. In this case, the current IBatt is plotted in units of a rate of charge c-rate (“current rate”). A discharging interval of the battery is denoted by the reference sign 201 and a charging interval of the battery is denoted by the reference sign 202. According to the method, the internal resistance Ri of the battery cells is determined directly from the voltage and current differences of two instants under consideration. In this case, in FIG. 2, the measurement instants for the respective measurements of the battery voltage UBatt are numbered with Roman numerals. The indices of the voltage and current values given below relate to said measurement instants. By way of example, the internal resistance for a discharge lasting 10 seconds results from evaluating the voltage and current values at the measurement instants (i) at t=0 s and (iii) at t=10 s to give
                              R                                    i              ⁢              _                        ⁢            10            ⁢                                                  ⁢            s_dch                          =                                                            U                III                            -                              U                I                                                    I              III                                .                                    (        1        )            
The internal resistance for a charging process lasting 10 seconds can correspondingly be calculated by evaluation of the voltage and current values at the instants (vi) at t=58 s and (viii) at t=68 s according to (2):
                              R                                    i              ⁢              _                        ⁢            10            ⁢                                                  ⁢            s_ch                          =                                                            U                VIII                            -                              U                VI                                                    I              VIII                                .                                    (        2        )            
This method is used when characterizing the cells on a testing bench. A disadvantage of the method is that it is difficult to comprehensively reproduce on the testing bench all conceivable real driving situations which could occur, for example, during operation of the battery in a motor vehicle. Therefore, the thus determined values for the internal resistance are useable for further processing in a battery management system in a vehicle only conditionally or only with the restriction of limited accuracy.